tocchapterIntroduction
On September 15, 2015, LIGO [1]} and Virgo [2]} large interferometers made the first direct detection of Gravitational Waves (GWs) emitted by a binary coalescing black-hole system [3]}. This event was not only a direct proof of the existence of gravitational waves and of binary coalescing black-hole systems, but it also gave us a new instrument to probe our Universe and to test General Relativity (GR) in the strong field regime. Since then, more than 90 events have been detected by the LIGO, Virgo, and Kagra[4]} collaborations during the first three operative runs O1,O2,O3 [5]}, [6]}, [7]}, [8]}, in the upcoming years more detection runs have been planned, and exciting outcomes are expected.
